How about this one? :)
http://www.mangago.zone/read-manga/kusatta_kyoushi_no_houteishiki/
OMG, so many:
Fake
http://www.mangago.zone/read-manga/kusatta_kyoushi_no_houteishiki/
Dear Green
New York, New York
Banana Fish
Eroica (series like this one http://www.mangago.zone/read-manga/eroica_yori_ai_wo_komete/ )
Old is Gold :)
http://www.mangago.zone/read-manga/love_mode/
http://www.mangago.zone/read-manga/complex_manda_ringo/
http://www.mangago.zone/read-manga/kimi_wa_boku_wo_suki_ni_naru/
http://www.mangago.zone/read-manga/wish/
http://www.mangago.zone/read-manga/moon_and_sandals/ + other mangas by Yoshinaga Fumi are also recommended.
http://www.mangago.zone/read-manga/dear_myself/
http://www.mangago.zone/read-manga/boys_next_door/
http://www.mangago.zone/read-manga/coponicus_no_kokyuu/ (2002 but still recommending it as you said you liked deep dark plots like Zankoku)
I also 2nd New York New York. 1 of the rare joy among long old mangas.
Any good yaoi/shounen ai manga that's pretty old? Like around the 80's-early 2000's? You'll know by the art if it's old. I started to like reading recently.